This invention generally relates to apparatus and methods for waveform equalization coefficient generation for use in multivalued digital microwave communication.
In recent years, there has been a tendency towards the multivaluing of modulation/demodulation of the digital microwave communication type for the efficient use of frequency. In addition to modulation techniques such as QPSK and 16QAM, the use of 64QAM and 256QAM has just started.
As the multivaluing of modulation/demodulation advances, effects due to signal distortion or the like occurring in communication paths become relatively serious. Accordingly, technology, having the ability to guarantee that correct signals are received at the receiving side, plays an important part, and automatic adaptive equalizers to achieve transmission path equalization on the receiving side have been proposed.
Transmission path equalization is first described below.
FIG. 16 shows a transmission path and an equalization model thereof. As can be seen from FIG. 16, signals from the transmitter, which vary according to the characteristics of a transmission path, are received together with noise. On the receiving side, an equalizer, serially arranged before the receiver, equalizes a received signal X0 to a desirable signal Z0 for the receiver. When noise is negligible, it is sufficient to employ an equalizer opposite in characteristic to the transmission factor of the transmission path. On the other hand, when noise is great to a certain extent, it is necessary to perform design of an equalizer in consideration of noise. A practically-used equalizer is formed by a digital filter, which is called a digital equalizer.
FIG. 17 is a block diagram of a digital filter. In FIG. 17, X0 is an input received signal applied from through a transmission path. X1 to Xm are received signals having a delay, produced by each delay element, with respect to X0. C0 to Cm are equalization coefficients. X0 is multiplied by C0 by multiplier. Likewise, X1-Xm are multiplied by C1-Cm, respectively. The multiplying results are summed together by adder and the sum is provided as an equalization signal, Z0.
A mechanism of multiplying together a delay signal and an equalization coefficient, employed in a digital filter, is known in the art as a tap. By summing together products produced by respective taps, Z0 is obtained. The equalization coefficients C0-Cm best suited for signal restoration are then calculated by a waveform equalization coefficient generator.
An algorithm of generating equalization coefficients is now described below.
As previously described, signals from a transmitter, which vary according to the transmission path characteristic, arrive at a receiver with noise. If the characteristic of a transmission path is constant, then it is sufficient to calculate the opposite transmission path characteristic and use a fixed equalization coefficient that implements the calculated opposite transmission path characteristic. However, in a system in which the effects of noise and characteristics change with time, it is necessary to carry out sequential equalization coefficient updating according to the state of received signals. An automatic adaptive type algorithm is used in equalization coefficient updating. Practically, based on an equalization coefficient in the preceding state, the subsequent equalization coefficient is computed. In this case, a specific evaluation index is set, and equalization coefficient updating processing is carried out in order that the set value is minimized. A typical algorithm of such a type is the LMS (least means square) algorithm.
In the LMS algorithm, a means square error is used as an evaluation index for equalization coefficient. More specifically, the equalization coefficient is given by
Cn+1,m=Cn,mxe2x88x92xcex1xc3x97Xmxc3x97e0xe2x80x83xe2x80x83(Expression 1)
where n is the number of times the equalization coefficient is updated, m is an equalization coefficient""s tap number, e0 is Z0xe2x88x92x0 (x0 is a pre-transmission signal), and xcex1 is the step size.
If the signal Xm and the error data e0 are complex-expressed by
Xm=Xm(r)xe2x88x92jXm(i)
e0=e0(r)+je0(r)
where (r) represents the real part data and (i) represents the imaginary part data, then the following is obtained.
Xmxc3x97e0=(Xm(r)xc3x97e0(r)+Xm(i)xc3x97e0(i))+j(Xm(r)xc3x97e0(i)xe2x88x92Xm(i)xc3x97e0(r))
Accordingly, Expression (1) becomes the following.
Cn+1,m(r)=Cn,m(r)xe2x88x92xcex1xc3x97(Xm(r)xc3x97e0(r)+Xm(i)xc3x97e0(i)) xe2x80x83xe2x80x83(Expression 2)
Cn+1,m(i)=Cn,m(i)xe2x88x92xcex1xc3x97(Xm(r)xc3x97e0(i)xe2x88x92Xm(i)xc3x97e0(r)) xe2x80x83xe2x80x83(Expression 3)
However, in actual transmission systems, the pre-transmission signal, x0, is unknown at the receiving side and therefore cannot be used to calculate the error data e0. For this reason, it is designed such that estimation for pre-transmission signals is performed on the receiving side and waveform equalization processing is carried out using the estimated value as a reference signal. This is called a blind algorithm.
Practically, it very hard to understand the fact that updating by a blind algorithm, when it is repeated several thousand times under specific conditions, results in equalization coefficient convergence and signal waveform equalization is accomplished. A commonly-used waveform equalization coefficient generator performs arithmetic operations as shown in Expressions (2) and (3) for equalization coefficient updating.
However, there are the following problems with conventional waveform equalization coefficient generators.
When performing equalization coefficient updating according to Expressions (2) and (3), it is necessary to prepare error data for respective signal data. This enormously increases the storage capacity of memory necessary for storing the error data and thereby increases the size of circuitry.
In addition to the above-described problem, there is another problem that many arithmetic units are required for performing equalization coefficient updating. For instance, six multipliers, and four adders or subtracters are necessary. A conventional waveform equalization coefficient generator becomes large in circuit size and consumes more electric power.
Bearing in mind the above-described problems with the prior art techniques, the present invention was made. It is therefore a general object of this invention to provide an improved waveform equalization coefficient generator which is smaller in circuit size and which consumes less electric power in comparison with conventional ones.
The present invention provides the following waveform equalization coefficient generators.
In a digital equalizer for waveform equalizing a signal modulated by a multivalued QAM (quadrature amplitude modulation) technique and transmitted and a pre-transmission signal, the present invention provides a waveform equalization coefficient generator for generating equalization coefficients for respective taps of digital filters which perform signal waveform equalization wherein said waveform equalization coefficient generator carries out updating of an equalization coefficient by means of separate updating of real part data and imaginary part data of said equalization coefficient and wherein a common arithmetic unit is used to perform arithmetic operations for updating said real part data and said imaginary part data by adaptively selecting data required in said arithmetic operations.
Such arrangement achieves a considerable simplification in apparatus configuration and thereby reduces not only the size of circuit but also the consumption of electric power.
The above-described waveform equalization coefficient generator comprises:
(a) an error data generation unit for generating error data necessary to update an equalization coefficient to a received signal; and
(b) a complex arithmetic unit for performing, based on said received signal and error data output from said error data generation unit, updating of said equalization coefficient by means of separate updating of real part data and imaginary part data of said equalization coefficient;
wherein:
said error data generation unit carries out generation of said error data by means of separate generation of I-axis direction error data and Q-axis direction error data; and
said complex arithmetic unit adaptively selects between real part data and imaginary part data of said received signal and adaptively selects between said I-axis direction error data and said Q-axis direction data, to perform arithmetic operations for updating said real part data and imaginary part data of said equalization coefficient by means of a common arithmetic unit.
The above-described complex arithmetic unit includes:
(a) data selection means for selecting between real part data and imaginary part data of a pre-updating equalization coefficient, selecting between real part data and imaginary part data of a received signal, and selecting between I-axis direction error data and Q-axis direction error data output from said error data generation unit, according to a data selection control signal applied, and for providing the selected data; and
(b) sum-of-products means for multiplying together one of said real part data and said imaginary part data of said received signal that is output from said data selection means and one of said I-axis direction error data and said Q-axis direction error data that is output from said data selection means and for summing together data obtained from results of said multiplying operation, to update either said real part data or said imaginary part data of said pre-updating equalization coefficient.
In a digital equalizer for waveform equalizing a signal modulated by a multivalued QAM (quadrature amplitude modulation) technique and transmitted and a pre-transmission signal, the present invention provides a waveform equalization coefficient generator for generating equalization coefficients for respective taps of digital filters which perform signal waveform equalization,
said waveform equalization coefficient generator comprising:
(a) an error data generation unit for generating, to a received signal, error data necessary to update an equalization coefficient with a STOPandGO algorithm that is an LMS (Least Means Square) algorithm using a STOPandGO function; and
(b) a complex arithmetic unit for performing, based on said received signal and said error data output from said error data generation unit, equalization coefficient updating;
said error data generation unit including:
(c) a coordinate decision unit for finding coordinate data indicative of a location of a multivalued QAM phase diagram occupied by a received signal and for providing, based on said coordinate data found, either a stop data signal indicative of the interruption of the output of error data or address data corresponding to said coordinate data;
(d) storage means for storing error data corresponding to each coordinate data and for providing error data stored at an address identified by said address data output from said coordinate decision unit; and
(e) a data selection unit for either providing a predetermined value in the presence of said stop data signal from said coordinate decision unit or providing said error data output from said storage means in the absence of said stop data signal.
It is preferred that arithmetic operations for equalization coefficient updating are brought to a halt in said complex arithmetic unit when said coordinate decision unit provides said stop data signal.
In a digital equalizer for waveform equalizing a signal modulated by a multivalued QAM (quadrature amplitude modulation) technique and transmitted and a pre-transmission signal, the present invention provides a waveform equalization coefficient generator for generating equalization coefficients for respective taps of digital filters which perform signal waveform equalization,
said waveform equalization coefficient generator comprising:
(a) an error data generation unit for generating, to a received signal, error data necessary to update an equalization coefficient with a STOPandGO algorithm that is an LMS (Least Means Square) algorithm using a STOPandGO function; and
(b) a complex arithmetic unit for performing, based on said received signal and said error data output from said error data generation unit, equalization coefficient updating;
said error data generation unit including:
(c) a coordinate data selection unit for determining whether coordinate data indicative of a location of a multivalued QAM phase diagram occupied by said received signal is positive or negative, wherein, when said coordinate data is determined to be positive, said coordinate data is provided intact from said coordinate data selection unit as coordinate data of judgement while on the other hand, when said coordinate data is determined to be negative, said coordinate data is bit-inverted and then provided from said coordinate data selection unit as coordinate data of judgement, and wherein said coordinate data selection unit provides an inversion indication signal indicative of whether said judgement coordinate data has undergone bit inversion processing;
(d) an error evaluation data storage unit for storing a plurality of error evaluation data corresponding to positive coordinate data, for selecting, from said plurality of error evaluation data, error evaluation data corresponding to said judgment coordinate data output from said coordinate data selection unit, and for providing the selected error evaluation data; and
(e) an error data selection unit;
when said coordinate data selection unit provides said inversion indication signal indicative of the absence of bit inversion, said error data selection unit then providing, in intact form, said error evaluation data output from said error evaluation data storage unit as error data;
when said coordinate data selection unit provides said inversion indication signal indicative of the presence of bit inversion, said error data selection unit then bit-inverting said error evaluation data and providing said bit-inverted error evaluation data as error data.
As a result of such arrangement, the error data generation unit generates, based on error evaluation data corresponding to positive coordinate data previously stored in the error evaluation data storage unit, error data to a received signal. In other words, data required to be pre-stored for error data generation are only error evaluation data corresponding to positive coordinate data. A considerable reduction of the capacity of storage required by the error data generation unit can be realized, which makes it possible to implement a waveform equalization coefficient generator with downsized circuitry.
In a digital equalizer for waveform equalizing a signal modulated by a multivalued QAM (quadrature amplitude modulation) technique and transmitted and a pre-transmission signal, the present invention provides a waveform equalization coefficient generator for generating equalization coefficients for respective taps of digital filters which perform signal waveform equalization,
said waveform equalization coefficient generator comprising:
(a) an error data generation unit for generating, to a received signal, error data necessary to update an equalization coefficient with a STOPandGO algorithm that is an LMS (Least Means Square) algorithm using a STOPandGO function; and
(b) a complex arithmetic unit for performing, based on said received signal and said error data output from said error data generation unit, equalization coefficient updating;
said error data generation unit including:
(c) a reference data storage unit for storing:
Sato error reference data indicative of a coordinate for a Sato error reference point in a multivalued QAM phase diagram;
coordinate decision reference data indicative of a slice level value in said multivalued QAM phase diagram;
error decision reference data indicative of a coordinate for a signal reference point in said multivalued QAM phase diagram;
(d) an LMS error generation unit for determining, based on coordinate data indicative of a location of said multivalued QAM phase diagram occupied by said received signal as well as on coordinate decision reference data output from said reference data storage unit, which of regions in said multivalued QAM phase diagram defined by respective slice levels said received signal belongs to, and for generating, based on error decision reference data output from said reference data storage unit indicative of a coordinate for a signal reference point in said determined region, LMS error data to said received signal;
(e) a Sato error generation unit for generating, based on Sato error reference data as well as on coordinate decision reference data output from said reference data storage unit, Sato error data to said received signal; and
(f) an error data selection unit for receiving said LMS error data generated by said LMS error generation unit and a sign bit of said Sato error data generated by said Sato error generation unit,
when a sign bit of said LMS error data and said sign bit of said Sato error data agree, said error data selection unit selecting said LMS error data and providing said LMS error data as error data;
when said sign bits disagree, said error data selection unit selecting data having a predetermined value and providing said data as error data.
As a result of such arrangement, the error data generation unit generates, based on the Sato error reference data, coordinate decision reference data and error decision reference data previously stored in the reference data storage unit, error data to a received signal. In other words, data required to be pre-stored for error data generation are only reference data of respective types. A considerable reduction of the capacity of storage required by the error data generation unit can be realized, which makes it possible to implement a waveform equalization coefficient generator with downsized circuitry.